Homozygosity for the CCR-5(32 gene is linked to a high degree of protection from HIV-1 infection. The CCR-5(32 allele carries a stable deletion of 32 bp which results in a truncated protein and in a frameshift that leads to a new sequence coding for a 31 AA carboxy terminus. The CCR-5delta32 allele is present in a substantial percentage (approximately 20%) of Caucasians, and could be the result of a genetic drift in the distant past that provided a selective advantage to undefined challenges. The genetic stability of CCR-5delta32, together with the absence of genetic polymorphisms at the boundaries of the 32 bp deletion, suggest that the gene product of the CCR-5delta32 allele may exert a yet undefined protective function. The resistance to HIV-1 infection observed in CCR-5delta32+/+ subjects has been explained as resulting from the absence of a functional CCR-5 protein which acts as a co-receptor for some HIV-1 strains. Alternatively, the gene product of the CCR-5delta32 allele could interfere with critical steps of the HIV-1 life cycle, resulting in a broad protection against different viral strains. To test this possibility, we constructed eukaryotic plasmids expressing the wild-type CCR-5 or the CCR-5delta32 allele, and used these to generate cell lines from CCR-5delta32+/+ subjects which express the wild type CC0R-5 allele, together with CCR-5+/+ cell lines expressing the CCR-5delta32 allele. These cells were challenged with different HIV-1 isolates. We found that CCR-5delta32+/+ cells are at least one log less sensitive to infection by different HIV-1 strains. Consistent with this finding, the exogenous expression of CCR-5delta32 in wild-type cells resulted in a dramatic decrease in viral production, suggesting that the CCR-5delta32 protein can function as a negative transdominant of viral expression. In transient expression experiments, the CCR-5(32 protein down-regulated both the constitutive and the Tat-induced expression of a LTR-Luciferase reporter plasmid, indicating that the CCR-5delta32 protein may interfere with transcription of the HIV-1 genome. By flow cytometry and by confocal microscopy, the CCR-5delta32 protein was found in the cytoplasm, possibly associated with the cytoskeleton and with the inner cell membrane. These findings may shed some light on the mechanisms of resistance to HIV-1 infection of CCR-5delta32+/+ individuals, and may lead to the development of new therapeutic tools to prevent the establishment of HIV-1 infection and to control disease progression in HIV-1 infected subjects.